1. Field of the Invention
The present invention generally relates to an optical module, particularly to an optical module which is densely space-division multiplexed by using a microlens array, and a method for assembling the optical module.
2. Description of the Related Art
A conventional optical module of this type has been disclosed in Japanese patent publication No. 2-123301. This conventional optical module comprises, as shown in FIG. 1, a planar microlens array 60 consisting of a planar transparent substrate having microlenses 61 formed in a surface thereof. Recesses 65 are formed in a surface opposite to the lens-formed surface of the array 60, each of recesses 65 being aligned with the center position of a corresponding microlens 61. An optical element to be optically coupled to the microlens 61 is an optical fiber 63, for example. The end core portion of an optical fiber is processed by a selective etching to form a micro fitting convex portion 66. When an optical fiber is fixed to the array 60, the convex portion 65 of an optical fiber is fitted to the recess 64 through a guide hole 64 which is opened in a layer 62 of resin such as polyimide adhered to the recess-formed surface of the array 60, the guide hole serving as a guide for inserting the end of an optical fiber to a recess. According to the conventional optical module described above, an alignment can easily be conducted by inserting the convex portion 66 of an optical fiber into the fitting recess 65 to fix it thereto, instead of an active alignment (i.e., light is guided into an optical fiber and the position of the optical fiber is regulated so as to maximize light coupled to a microlens).
The conventional optical module described above causes the following problems in such a case that the optical module is used to combine especially with a planar optical element. That is, when the planar optical element is a planar transmission optical element module such as a liquid crystal switch, a microlens optical system must be constructed by a collimate optical system having an infinite conjugate ratio. On the contrary, a microlens optical system is required to be constructed by a reducing image optical system having a finite conjugate rate, when the planar optical element is an optical element module which connects a light-emitting element such as a laser array to an optical fiber. It should be noted that the conjugate ration means the ratio of an object distance to an image distance.
In the case of an optical module coupled to optical fibers, a microlens optical system is required to be constructed by a unity magnification image optical system, while in the case of an optical module coupled to a planar optical element such as a photo-detector array having a light-receiving area larger than a mode field diameter of an optical fiber, a microlens optical system is required to be constructed by a magnification image optical system.
In order to satisfy these requirements, planar microlens arrays each having a different focal length of microlens are prepared separately so that an optimal conjugate ratio may be obtained for respective application in the conventional optical module, or the thickness of a substrate of planar microlens array is regulated to obtain an optimal conjugate ratio. As a result, the number of kinds of planar microlens arrays will be increased. Also, when the optical characteristic of an optical module is regulated in an experimental environment for example, it is often required to remake a planar microlens array. In such a case, an efficient development and early implementation of optical modules will be disturbed.
Accordingly, the object of the present invention is to provide an optical module wherein any conjugate ratio thereof in an optical axis direction may be easily realized.
Another object of the present invention is to provide an optical module wherein the time duration for alignment may be shortened by using a passive alignment technique (i.e., a method of alignment based on an accuracy of a mechanical dimension).
A further object of the present is to provide an optical module wherein a large degree of freedom may be obtained for any optical design.
A further object of the present invention is to provide a method for assembling the optical module described above.
Therefore, a first aspect of the present invention is an optical module comprising: a planar microlens array having a plurality of microlenses formed in at least one surface thereof; a planar transparent substrate for adjusting a conjugate ratio of the optical module, one surface of the transparent substrate being adhered to one surface of the planar microlens array, and a plurality of micro fitting recesses being formed in the other surface of the transparent substrate with each of the recesses being aligned to each of the microlenses; and a guide substrate for optical fibers, the guide substrate being adhered to the other surface of the transparent substrate, and the guide substrate having a plurality of micro guide holes opened therein with each of the guide holes being aligned to each of the recesses.
A second aspect of the present invention is a method for assembling an optical module comprising the steps of: preparing a planar microlens array having a plurality of microlenses formed in at least one surface thereof; adhering one surface of a planar transparent substrate for adjusting a conjugate ratio of the optical module to one surface of the planar microlens array, the transparent substrate having a plurality of micro fitting recesses formed in the other surface thereof, in such a manner that the center position of each of the recesses is aligned to the center position of each of the microlenses; and adhering one surface of a guide substrate for optical fibers to the other surface of the transparent substrate, the guide substrate having a plurality of micro guide holes opened therein, in such a manner that the center position of each of the guide holes is aligned to the center position of each of the recesses.